As illustrated in FIG. 2, a known guide holder device comprises a metallic holder (1), into which a diamond guide member (2) is inserted. The guide member (2) has a cylindrical hole (4), through which a wire electrode (3) is passed. As the wire electrode (3) is very fine and the hole (4) has a very small diameter of at most 0.2 mm, it is a difficult and time-consuming operation to insert the wire electrode into the hole (4). Furthermore, the known guide holder device has also such a shortcoming that an electric corrosion occurs at the metallic holder (1).
If a sapphire guide member (2'), as shown in FIG. 3, is employed instead of the diamond guide member (4), then it is practically possible to form the both ends of the hole (4) into the shape of a truncated cone (4a) (4b), because sapphire has a hardness lower than that of diamond. However, the wear rate of the wall of the hole (4) of the sapphire guide member (2') is considerably higher than that of the diamond guide member.
It is an object of the invention to provide an improved guide holder of an EDM apparatus, said guide holder comprising a diamond guide member and not having any shortcomings as mentioned above.